1. Field
This application generally relates to the fabrication and utilization of micron-scale structures. More particularly, this application relates to making an electrical connection using micron-scale structures.
2. Related Art
Current integrated circuit mounting techniques use solder and a protective coating for electrical interfacing (solder pads and pin-outs) and mechanical stability (underfill). Traditionally, as integrated circuit manufacturing processes and circuit architectures advance, pin-out numbers increase, and silicon die areas can potentially decrease. Larger pin-out numbers are desirable for both increased potential inputs/outputs and power supply. Smaller die areas create more compact devices and decrease production costs per die. The trend towards larger pin-out numbers and smaller die areas creates trends toward smaller solder pads for interfacing with substrates.
Many modern integrated circuits use a technique known as controlled collapse chip connection (C4) to interface integrated circuits with a substrate. C4 begins with the formation of an array of small solder balls on the solder pads of an integrated circuit. The integrated circuit-solder ball assembly is then flipped over onto the substrate such that each solder ball lines up with its intended solder pad on the substrate. The solder balls are then re-flowed to ensure bonding to substrate solder pads, and the final solder joint shape is dictated by the surface tension of the particular solder employed and the weight of the integrated circuit pressing down on the molten solder.
Each soldering step used in the C4 process, however, uses temperatures high enough to potentially damage delicate integrated circuits. Also, decreasing solder ball size results in decreasing distance between silicon die and substrate, thus decreasing final solder joint thickness. Additionally, due to a coefficient of thermal expansion mismatch between silicon die and substrate, solder joints are subject to considerable stress during temperature change and fatigue during temperature cycle. Such mechanisms may induce cracks in solder joints, leading to solder joint failure and integrated circuit malfunction.